1. Field of the Invention
The present invention relates to a timing element, in particular, to an energetic linear timing element, such as timing elements used in an initiator, such as a detonator or other explosive initiating device, for delaying initiation of the explosive output charge in the initiator.
2. Related Art
A timing element (also known as a fuse) is typically located within an initiator and is interposed between a signal transmission input line and an output charge, thereby connecting the input line and the output charge in initiation signal communication. Timing elements are used to delay the travel of an initiation signal between the signal transmission input line and the detonator output charge for a predetermined period of time (the “delay interval” or “burn time”) that may range from 9 milliseconds (“ms”) to 900 ms, typically, 350 ms to 500 ms. Conventional timing elements include “drawn” timing elements and “rigid” timing elements, both of which comprise a pulverulent delay composition disposed within a metal sheath. The pulverulent delay composition may comprise pyrotechnic material comprising any one or more of the following mixtures of fuels and oxidizers: silicon and lead dioxide (PbO2); silicon and red lead oxide (Pb3O4); silicon, red lead oxide (Pb3O4) and antimony trisulfide (Sb2S3); tungsten, potassium perchlorate (KClO4) and barium chromate (BaCrO4); molybdenum and potassium perchlorate (KClO4); and others, and mixtures thereof. In a conventional drawn timing element, the sheath comprises a material such as lead, pewter, aluminum or other suitable ductile metal that may readily be deformed by pressure or crimping. Typically, a hollow tube of a soft, malleable metal such as lead or pewter is filled with a pyrotechnic material, and the filled tube is then passed through a series of reducing dies to decrease the diameter of the sheath and compress the pyrotechnic material. The burn rate of the timing element is dependent upon the composition and resulting density of the pyrotechnic material, and on the length of the timing element. In a rigid timing elements, the sheath comprises a hard, non-malleable metal of fixed length, e.g., steel or zinc, loaded with pyrotechnic material and, possibly, an organic binder. The loaded material is pressed to a fixed height and density to provide a selected burn time.
An initiator comprises a shell into which an output charge (or base charge) is deposited. The output charge provides the output signal of the initiator. The output charge is generally pressed into the shell, and then other components, optionally including a timing element, are pressed into the shell over the base charge. An initiating means, i.e., an input signal line (detonating cord, fuse cord, shock tube, etc.), is then secured to the shell with a seal to prevent contaminants from entering the shell. Pressing is performed with a pin, the end of which may be crowned, pointed or slightly tapered. Initiators include detonators, which provide explosive output signals and pyrotechnic initiators, which provide pyrotechnic output signals.
A “signal transmission tube” is a hollow plastic (polymer) tube having a reactive material on the interior surface thereof and being suitable for use in transmitting a detonation signal through the tube by ignition of the reactive material. The defined term embraces shock tubes of the type disclosed in U.S. Pat. Nos. 4,328,753 and 4,607,573, which have a coating of pulverulent high brisance explosive material such as PETN (pentaerythrite tetranitrate), RDX (cyclotrimethylenetrinitramine) (also known as Cyclonite or Hexogen), HMX (cyclotetrame-thylenetetranitramine) (Homocyclonite or Octogen) or TNT (2,4,6-trinitrotoluene) on the interior wall of the tube, and low velocity signal transmission tubes of the type disclosed in U.S. Pat. No. 5,257,764, which have deflagrating material on the interior surface of the tube. Deflagrating materials include silicon/red lead (Si/Pb3O4), molybdenum/potassium perchlorate (Mo/KClO4), tungsten/potassium perchlorate (W/KClO4), titanium hydride/potassium perchlorate (TiH2/KClO4) and zirconium/ferric oxide (Zr/Fe2O3). Other suitable deflagrating compositions are boron/red lead (B/Pb3O4), titanium/potassium perchlorate (Ti/KClO4), zirconium/potassium perchlorate (Zr/KClO4), aluminum/potassium perchlorate (Al/KClO4), zirconium hydride/potassium perchlorate (ZrH2/KClO4), manganese/potassium perchlorate (Mn/KClO4), and the like. In both kinds of signal transmission tubes, the tube may be formed from an extruded synthetic polymeric material such as EAA (ethylene/acrylic acid copolymer), EVA (ethylene vinyl acetate) or a SURLYN™ such as SURLYN™ 8940, an ioniomer resin available from E. I. DuPont de Nemours Company, low density polyethylene (LDPE), linear low or medium density polyethylene, linear low, medium and high density polyester and polyvinylidene chloride (PVC), and suitable blends or polymer alloys of such materials.
U.S. Pat. No. 5,681,904 issued Oct. 28, 1997 to Anthony P. Manzara discloses a polymer material comprising a glycidyl azide polymer, either as a plasticizer or a polyol, cross-linked with a multifunctional acrylate (column 2, line 9 through line 14 and column 3, line 34 through line 41) or polyisocyanate. The hardness and burning properties of the polymer material are determined by the particular multifunctional acrylate, and amount thereof, which is cross-linked with the glycidyl azide polymer (column 8, line 44 through line 65). The polymer material may be used in a wide variety of applications, e.g., as a high energy material, as a binder in an explosive material or a rocket propellant, or with a gas-generating material used in an air bag apparatus (column 2, line 16 through line 24).